Analysis and design of multilayer frequency selective surfaces
Structures that include more than one layer of Frequency Selective Surfaces (FSS) are an attractive feature in applications with stringent performance requirements. They provide extra flexibility with regard to the adjustment of the transmission responses which can in principle be insensitive to angle of incidence because of the increased bandwidth. This thesis deals with computer simulation and experimental assessment of a variety of multilayer FSS structures. The plane wave vector modal analysis technique is adapted for analysing the scattering from these multilayer FSS structures. The novelty in the plane wave modal analysis method lies in the fact that they can be applied to arbitrary lattice and element geometries. A novel super-resolution approach of analysing the scattering from FSS in cascade, with arbitrary lattice geometries of the two arrays is outlined. These type of structures exhibit multiresonant responses in a controlled manner. The problem of assigning different lattice geometries to the structure is addressed here by assigning the periodic fields adjacent to the arrays a common (or mutual) periodicity and by employing the convolution theorem to the modal (Floquet) sets that expands the tangential fields in each array. As a result, the spectral components of the Floquet mode coefficients from the various adjacent arrays are related to those of the common periodicity by means of a correlation function. This correlation function enables the spectral components of the Floquet mode sets expanding the tangential fields from any two adjacent arrays to be super-resolved from those of their common periodicity set. Once the convolution has been executed, application of electromagnetic boundary conditions are utilised, thus obtaining the coupled electric field integral equations. These integral equations relate the spectrums of the surface current densities to the various Floquet mode coefficients. The integral equations are in turn solved by the Method of Moment (MoM) technique for the unknown current coefficients from which the unknown transmission and reflection coefficients from the entire structure are obtained. A major assumption that is made in this technique for assigning a common periodicity lattice is that the ratio of lattice periodicities of any two adjacent arrays must be a rational number. The importance of the proposed technique lies in unlocking the complexities that exist when the scattered Aoquet modal coefficients from the arrays are related to the spectral components of the currents induced on the surfaces of the arrays, in the integral equation formulation. Furthermore, the proposed approach offers a computational advantage when applied to multilayer FSS structures, as it is invariant to the distance separating the arrays. A computer model based on this technique is developed for obtaining the prediction results. Various double layer FSS structures with arbitrary element types and lattice geometries of the arrays and with variable separation distances between the two layers are studied. The plane wave transmission coefficients of these multiresonant structures are computed with a v1ew to predict their radiation parameters. Extensive measurements are performed by using a purpose built experimental jig for mounting the structures, in an indoor anechoic chamber. The validity of the theoretical model is assessed by comparison with measurements from a variety of multilayer structures.